Congestion detection in a network interconnect

ABSTRACT

A method and system for detecting congestion in a network of nodes, abating the network congestion, and identifying the cause of the network congestion is provided. A congestion detection system may comprise a detection system, an abatement system, and a cauzation system. The detection system monitors the performance of network components such as the network interface controllers and tiles of routers to determine whether the network is congested such that a delay in delivering packets becomes unacceptable. Upon detecting that the network is congested, an abatement system abates the congestion by limiting the rate at which packets are injected into the network from the nodes. Upon detecting that the network is congested, a cauzation system may identify the job that is executing on a node that is the cause of the network congestion.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is related to U.S. patent application Ser. No.13/044,139, filed concurrently herewith, entitled “CONGESTION ABATEMENTIN A NETWORK INTERCONNECT” and U.S. patent application Ser. No.13/044,177, filed concurrently herewith, entitled “CONGESTION CAUSATIONIN A NETWORK INTERCONNECT”, each of which is incorporated by referenceherein in its entirety.

BACKGROUND

Massively parallel processing (“MPP”) systems may have tens of thousandsof nodes connected via a communications mechanism. Each node may includeone or more processors (e.g., an AMD Opteron processor), memory (e.g.,between 1-8 gigabytes), and a communications interface (e.g.,HyperTransport technology) connected via a network interface controller(“NIC”) to a router with router ports. Each router may be connected viaits router ports to some number of other routers and then to other nodesto form a routing topology (e.g., torus, hypercube, and fat tree) thatis the primary system network interconnect. Each router may includerouting tables specifying how to route incoming packets from a sourcenode to a destination node. The nodes may be organized into modules(e.g., a board) with a certain number (e.g., 4) of nodes and routerseach, and the modules may be organized into cabinets with multiple(e.g., 24) modules in each cabinet. Such systems may be consideredscalable when an increase in the number of nodes results in aproportional increase in their computational capacity. An examplenetwork interconnect for an MPP system is described in Alverson, R.,Roweth, D., and Kaplan, L., “The Gemini System Interconnect,” 2010 IEEEAnnual Symposium on High Performance Interconnects, pp. 83-87, MountainView, Calif., Aug. 18-20, 2010, which is hereby incorporated byreference.

The nodes of an MPP system may be designated as service nodes or computenodes. Compute nodes are primarily used to perform computations. Aservice node may be dedicated to providing operating system andprogramming environment services (e.g., file system services, externalI/O, compilation, editing, etc.) to application programs executing onthe compute nodes and to users logged in to the service nodes. Theoperating system services may include I/O services (e.g., access to massstorage), processor allocation services, log in capabilities, and so on.The service nodes and compute nodes may employ different operatingsystems that are customized to support the processing performed by thenode.

An MPP system may include a supervisory system comprising a hierarchy ofcontrollers for monitoring components of the MPP system as described inU.S. Patent Application No. 2008/0134213, entitled “Event NotificationsRelating to System Failures in Scalable Systems,” filed on Sep. 18,2007, which is hereby incorporated by reference. At the lowest level ofthe hierarchy, the supervisory system may include a controllerassociated with each node that is implemented as software that mayexecute on the node or on special-purpose controller hardware. At thenext lowest level of the hierarchy, the supervisory system may include acontroller for each module that may be implemented as software thatexecutes on special-purpose controller hardware. At the next lowestlevel of the hierarchy, the supervisory system may include a controllerfor each cabinet that also may be implemented in software that executeson special-purpose controller hardware. The supervisory system may theninclude other levels of controllers for groups of cabinets referred toas slices, groups of slices referred to as sections, and so on. At thetop of the hierarchy is a controller designated as the supervisorycontroller or system management workstation, which provides a view ofthe overall status of the components of the multiprocessor system. Thehierarchy of controllers forms a tree organization with the supervisorycontroller being the root and the controllers of the nodes being theleaf controllers. Each controller communicates between its parent andchild controller using a supervisory communication network that isindependent of (or out of band from) the primary system networkinterconnect. For example, the supervisory communication network may bea high-speed Ethernet network.

The controllers monitor the status of the nodes, network interfacecontrollers, and routers. A leaf controller (or node controller) maymonitor the status of the hardware components of the node and the systemservices executing on the node. The next higher level controller (modulecontroller or L0 controller) may monitor the status of the leafcontrollers of the nodes of the module, power to the module, and so on.The next higher level controller (cabinet controller or L1 controller)may monitor the status of the next lower level controllers, power to thecabinet, cooling of the cabinet, and so on.

FIG. 1 is a block diagram that illustrates an example controllerhierarchy of a supervisory system. The controller hierarchy 100 includesa root or supervisory controller 101. The supervisory controller is theparent controller for the section controllers 102. A section is agrouping of slices. Each section controller is a parent controller ofslice controllers 103. A slice is a grouping of cabinets. Each slicecontroller is a parent controller of cabinet controllers 104. A cabinetphysically contains the modules. Each cabinet controller is a parentcontroller of module controllers 105 within the cabinet. A module is aphysical grouping of a number (e.g., four) of nodes. Each modulecontroller is a parent controller of node controllers 106 on a module.The lines between the controllers represent the logical communicationspath between the controllers, which may be implemented as a supervisorycommunications network that is out of band from the primary systemnetwork interconnect, which is not shown in FIG. 1.

FIG. 2 is a block diagram that illustrates an example network interfaceand routing device of a network interconnect. A network device 200includes two network interface controllers (“NICs”) 210 and 211. Eachnetwork interface controller is connected via a HyperTransportconnection 220 or a HyperTransport connection 221 to a node (not shown).The network interface controllers are connected to a router 230 via anetlink 260. The network device also includes a supervisory component240 with a connection to a local controller 250. The packets from thenetwork interface controllers are routed via the netlink to the routerover a router input selected for load balancing purposes. The routerroutes the packets to one of 40 network connections. Each packet maycomprise a variable number of fixed-sized flow control units, referredto as “flits.”

FIG. 3 is a block diagram that illustrates the connections of an examplenetwork device. The network device 300 includes 40 router ports 301 forconnection to other routers in the network interconnect. The networkdevice includes four links of four ports each in the x and z directionsand two links of four ports each in the y direction.

FIG. 4 is a block diagram that illustrates the layout of an examplerouter. The router 400 comprises 48 tiles arranged into a matrix of sixrows and eight columns. The router provides 40 connections to thenetwork and eight connections to the network interface controllers viathe network link. Each tile 410 includes an input buffer 411, routinglogic 412, a row bus 413, row buffers 414, an 8×6 switch 415, a columnbus 416, output buffers 417, and output multiplexor 418. The packets arereceived at a tile via the router port connected to the input buffer andprocessed on a flit-by-flit basis by the routing logic. During eachcycle of the tile, the routing logic retrieves a flit (if available)from the input buffer and routes the flit via a line of the row bus toone of the row buffers of a tile in the same row. If that row buffer isfull, then the routing logic leaves the flit in the input buffer andrepeats the process during the next cycle. At each cycle, flits in therow buffers are routed via the 8×6 switch to an output buffer in a tilein the same column. During each cycle, the output logic sends a flitfrom an output buffer to the router port associated with that tile. Thetiles of the routers and the network interface controllers are referredto as “network components.”

Depending on the characteristics of the jobs executing on the computenodes, the network interconnect may not be able to transmit requestsfrom an originating node to a destination node and receive acorresponding response in a timely manner. For example, if many nodes(e.g., 999 nodes in a 1,000 node network) executing an execution threadof the job rapidly send requests to a single destination node alsoexecuting an execution thread of the job, then the buffers of the tilesthat lead to the destination node may become full. If the buffers arefull, then the routing logic of the tiles will spend cycles waiting forthe buffer to be no longer full. If the network interconnect cannotdeliver packets in a timely manner to even a single node, the speed atwhich all the jobs execute on the nodes of the network can be negativelyimpacted.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a block diagram that illustrates a controller hierarchy of thesupervisory system in some embodiments.

FIG. 2 is a block diagram that illustrates an example network interfaceand routing device of a network interconnect.

FIG. 3 is a block diagram that illustrates the connections of an examplenetwork routing device.

FIG. 4 is a block diagram that illustrates the layout of an examplerouter.

FIG. 5 is a block diagram that illustrates components of a supervisorycontroller of a congestion detection system in some embodiments.

FIG. 6 is a block diagram that illustrates components of a localcontroller of a congestion detection system in some embodiments.

FIG. 7 is a flow diagram that illustrates the processing of a congestiondetector component of a supervisory controller in some embodiments.

FIG. 8 is a flow diagram that illustrates the processing of a check tilecongestion criterion component of a supervisory controller in someembodiments.

FIG. 9 is a flow diagram that illustrates the processing of a collectperformance information component of a supervisory controller in someembodiments.

FIG. 10 is a flow diagram that illustrates the processing of a stalldetector component of a local controller in some embodiments.

FIG. 11 is a flow diagram that illustrates the processing of a generateNIC stall count component of a local controller in some embodiments.

FIG. 12 is a flow diagram that illustrates the processing of a check NICstall criterion component of a local controller in some embodiments.

FIG. 13 is a flow diagram that illustrates the processing of acongestion abatement component of a supervisory controller in someembodiments.

FIG. 14 is a flow diagram that illustrates the processing of a checkcongestion abatement criterion of a supervisory controller in someembodiments.

FIG. 15 is a flow diagram that illustrates the processing of acongestion abatement component of a local controller in someembodiments.

FIG. 16 is a flow diagram that illustrates the processing of an identifyjob causing congestion component of a supervisory controller in someembodiments.

FIG. 17 is a flow diagram that illustrates the processing of an identifysuspect jobs component of a supervisory controller in some embodiments.

FIG. 18 is a flow diagram that illustrates the processing of a check forguilty job component of a supervisory controller in some embodiments.

FIG. 19 is a flow diagram that illustrates the processing of a sendejection count component of a local controller in some embodiments.

DETAILED DESCRIPTION

A method and system for detecting congestion in a network of nodes,abating the network congestion, and identifying the cause of the networkcongestion is provided. A congestion detection system may comprise adetection system, an abatement system, and a causation system. Thedetection system monitors the performance of network components, such asthe network interface controllers and tiles of routers, to determinewhether the network is congested such that a delay in delivering packetsbecomes unacceptable. In some embodiments, the detection system collectsperformance measurements for each network component for everymeasurement period (e.g., one second). For example, the detection systemmay collect the number of flits routed by the routing logic of each tileduring the measurement period and the number of cycles during themeasurement period during which a flit was available (i.e., in the inputbuffer) but could not be routed because a destination buffer was full.The detection system determines from the collected performancemeasurements whether each network component is stalled during ameasurement period using a stall criterion. For example, the stallcriterion for a tile may be based on a ratio derived from the number offlits forwarded by the tile during a measurement period and the numberof clock cycles (i.e., tile periods) during the measurement period inwhich a flit was delayed (i.e., available, but could not be routed) atthe tile. The detection system may determine whether the network iscongested during a measurement period using a network congestioncriterion. For example, the network congestion criterion may specify anumber or a percentage of tiles. If more than that number or percentageof tiles are stalled during a measurement period, then the detectionsystem indicates that the network is congested. In some embodiments, alocal controller associated with each network component implements aportion of the detection system and determines whether the networkcomponent is stalled during a measurement period. Each local controllermay send via an out-of-band network connection information about stallednetwork components to a supervisory controller. The supervisorycontroller determines, based on the information received from multiplelocal controllers, whether the network is congested.

Upon detecting that the network is congested, an abatement system abatesthe congestion by limiting the rate at which packets are injected intothe network from the nodes. The abatement system limits the rate bysending to each local controller an indication that the network iscongested. Each local controller then limits the rate at which packetsfrom the nodes (e.g., compute nodes) that it controls are injected intothe network. Because the rate at which packets are injected into thenetwork is limited, the network components are able to eventuallyalleviate the congestion. Various techniques may be used to limit therate at which packets are injected into the network. In someembodiments, the local controller may use a “pulse width modulation”technique to limit the injection rate by limiting the time during whicha network interface controller can inject packets into the network to afraction of each measurement period or other period. In otherembodiments, the abatement system may require the packets to be spacedapart in time to ensure that the limited packet injection rate is notexceeded. The abatement system may also limit the rate of packetinjection by only allowing a limited number of requests to beoutstanding (i.e., for which a response has not been received) at thesame time. In some embodiments, a supervisory controller, upon detectingnetwork congestion, sends via the out-of-band network connection anabatement notification to each local controller associated with eachnetwork interface controller. The local controllers then effect thelimiting of the injection rate of packets into the network by thenetwork interface controller while the abatement is in effect.

Upon detecting that the network is congested, a causation system mayidentify the job that is executing on a node that is the cause of thenetwork congestion. The causation system may monitor the ejection rateat which packets are ejected from the network to a node duringabatement. The causation system may determine that the node whoseejection rate satisfies a congestion cause criterion may be executingthe job that is causing the network congestion. For example, thecongestion cause criterion may specify the node with the overall highestejection rate over a sequence of measurement periods or other periods.The causation system determines that a job executing on a node thatsatisfies the congestion cause criterion is the cause of the networkcongestion. The execution threads of such a job may be executing onmultiple nodes and sending many requests to a single node that is alsoexecuting an execution thread of that job. In some embodiments, asupervisory controller, upon detecting network congestion, analyzesejection rate information received from local controllers collected fromthe network interface controllers to determine whether the nodes satisfythe congestion cause criterion. The local controllers may send ejectionrate information to the supervisory controller via an out-of-bandnetwork connection. A job may comprise a single computer program ormultiple computer programs.

In some embodiments, the congestion detection system may attempt toidentify the job causing congestion by selective abatement of nodes.Prior to starting any abatement, the congestion detection system mayidentify candidate jobs as likely causing the congestion. For example,jobs executing on nodes with the highest congestion may be the candidatejobs. The congestion detection system then sends to the localcontrollers of the nodes executing the candidate jobs instructions tolimit packet injection. If congestion then abates, the congestiondetection system may assume that one or more of the candidate jobs isactually causing the congestion. If congestion does not abate, then thecongestion detection system may then instruct all the nodes to limittheir rates of injection and use the causation system to identify thejob causing congestion.

FIG. 5 is a block diagram that illustrates components of a supervisorycontroller of a congestion detection system in some embodiments. Thesupervisory controller 500 includes a detection system 510, an abatementsystem 520, a causation system 530, a stall count store 540, an ejectionrate store 550, and a node/job store 560. The detection system includesa congestion detector component 511, a check NIC congestion criterioncomponent 512, a check tile congestion criterion component 513, and acollect performance information component 514. The congestion detectorcomponent may be executed every measurement period or other period or onan ad hoc basis based on another event (e.g., a signal from a localcontroller) to determine whether the network is congested based on thenumber of network components that are stalled as reported by the localcontrollers. The check NIC congestion criterion component determineswhether the network is congested based on the number of stalled networkinterface controllers. The check tile congestion criterion componentdetermines whether the network is congested based on the number ofstalled tiles. The collect performance information component collectsstall and ejection rate information from the local controllers andstores the information in the stall count store and ejection rate store.The stall count store may include an entry for each local controller foreach measurement period that indicates the number of network components(e.g., a count of tiles and a separate count of network interfacecontrollers) that were stalled during that period as indicated by thelocal controllers. The ejection rate store may contain entries for eachnode indicating the ejection rate of that node during each performanceperiod during abatement.

The abatement system includes a congestion abatement component 521 and acheck congestion abatement criterion component 522. The congestionabatement component may be executed every measurement period todetermine whether the rate at which packets are injected into thenetwork should be limited or on an ad hoc basis when congestion isdetected. The congestion abatement component invokes the checkcongestion abatement criterion component to determine whether congestionshould be abated. If congestion should be abated, the congestionabatement component notifies the local controllers and may indicate thedegree to which each local controller is to limit the rate of packetinjection into the network. This degree may be expressed in variousforms, such as a fraction of a period (i.e., “pulse width”) during whicheach local controller will allow packets to be injected into the networkor as a percentage of the maximum injection rate. The abatement systemmay limit the injection rate only of compute nodes and not service nodesso that the service nodes can still provide services (albeit possiblyslower because of the congestion) to the jobs executing at the computenodes.

The causation system includes an identify job causing congestioncomponent 531, an identify suspect jobs component 532, and a check forguilty job component 533. The identify job causing congestion componentmay be invoked periodically during abatement of all nodes (includingservice nodes) or on an ad hoc basis during abatement of all nodes. Theidentify job causing congestion component invokes the identify suspectjobs component to identify jobs executing on nodes whose ejection ratesatisfies an ejection rate threshold for a measurement period (e.g.,jobs that are misbehaving or have a design flaw). In some embodiments,the component may only monitor the ejection rates of compute nodesbecause jobs or programs of the service nodes may be assumed to behave.The ejection rate information may be collected and stored in theejection rate store. The node/job store contains a mapping of each nodeto the jobs executing on that node. The check for guilty job componentdetermines whether any of the suspected jobs satisfies the congestioncause criterion and indicates those jobs that are the likely cause ofthe network congestion. The causation system may in certaincircumstances incorrectly identify a job as the cause of congestion. Forexample, during abatement, a job may have the highest ejection rate at anode but the job may be executing on nodes very close to that noderesulting in very little impact on overall network congestion. Incontrast, another job may be flooding the network with packets sent tomany different nodes but none with a high ejection rate.

In some embodiments, the causation system may use a highest stall ratioalgorithm to identify a job that is causing congestion. This algorithmis based on a principle that the network components of a node executinga job with a high injection rate are likely stalled more frequently. Thestall ratio is the ratio of the number of flits whose injection to thenetwork was stalled and the number of flits injected into the network. Alocal controller may collect these numbers prior to abatement. Whenabatement starts, each local controller analyzes its counters toidentify a period in which the stall ratio was above a threshold stallratio and reports it to the supervisory controller. The supervisorycontroller then identifies a job whose nodes have a high stall ratio ascandidate job for causing congestion. The supervisory controller may usestall ratio as part of the criterion for identifying candidate jobs.

FIG. 6 is a block diagram that illustrates components of a localcontroller of a congestion detection system in some embodiments. Thelocal controller 600 includes a detection component 610, an abatementcomponent 620, and a causation component 630. The detection componentincludes a stall detector component 611, a generate NIC stall countcomponent 612, a check NIC stall criterion component 613, a generatetile stall count component 614, a check tile stall criterion component615, and a stall count store 616. The stall detector component invokesthe generate NIC stall count component and the generate tile stall countcomponent to generate the count of the stalled network components for ameasurement period. Each stall count component invokes the correspondingcheck stall criterion component to determine whether the correspondingnetwork component satisfies the corresponding stall criteria. The stalldetector component may store the stall count information in the stallcount store. The stall detector component sends the stall countinformation to the supervisory controller. The abatement componentincludes a congestion abatement component 621 that controls the limitingof the injection rate of packets into the network as instructed by thesupervisory controller. The causation component includes a send ejectioncount component 631 that collects packet ejection information and sendsthe information to the supervisory controller.

The devices on which the congestion detection system may be implementedmay include a central processing unit and memory and may include,particularly in the case of the system management workstation, inputdevices (e.g., keyboard and pointing devices), output devices (e.g.,display devices), and storage devices (e.g., disk drives).Computer-readable media includes computer-readable storage media anddata transmission media. The computer-readable storage media includesmemory and other storage devices that may have recorded upon or may beencoded with computer-executable instructions or logic that implementthe congestion detection system. The data transmission media is mediafor transmitting data using signals or carrier waves (e.g.,electromagnetism) via a wire or wireless connection. Various functionsof the congestion detection system may also be implemented on devicesusing discrete logic or logic embedded as an application-specificintegrated circuit. The devices on which the congestion detection systemis implemented computing devices.

The congestion detection system may be described in the general contextof computer-executable instructions, such as program modules, executedby one or more computers or other devices. Generally, program modulesinclude routines, programs, objects, components, data structures, and soon that perform particular tasks or implement particular abstract datatypes. Typically, the functionality of the program modules may becombined or distributed as desired in various embodiments.

FIG. 7 is a flow diagram that illustrates the processing of a congestiondetector component of a supervisory controller in some embodiments. Thecomponent may be passed an indication of the current measurement periodand determines whether the network is congested based on informationreceived from the local controllers. In block 701, the component invokesthe check NIC congestion criterion component. In block 702, thecomponent invokes the check tile congestion criterion component. Indecision block 703, if the network interface controllers or the tilessatisfy the congestion criterion, then the component indicates that thenetwork is congested in block 704, else the component indicates that thenetwork is not congested in block 705. The component then completes. Thecomponent may alternatively determine whether the network is congestedbased on a combination of information derived from both the networkinterface controllers and the tiles or from just the network interfacecontrollers or the tiles. For example, the component may detectcongestion when both the stall count of the network interfacecontrollers is above a certain threshold and the stall count of thetiles is above another threshold.

FIG. 8 is a flow diagram that illustrates the processing of a check tilecongestion criterion component of a supervisory controller in someembodiments. In some embodiments, the component determines that thenetwork is congested when the number of stalled tiles is greater than anupper threshold and determines that the network is no longer congestedwhen the number of stalled tiles is less than a lower threshold. Thecomponent may make this determination whenever a count of stalled tilesis received. The use of an upper and lower threshold provides ahysteresis effect to help prevent the component from toggling toorapidly between determinations of congestion and determinations of nocongestion. In this example embodiment, the component determines whetherthe tile stall information for the current measurement period indicatesthat the network appears to be congested. The component then determineswhether the network is actually congested based on analysis of thecongestion pattern over multiple measurement periods. In decision block801, the component determines whether the tile stall counts for thecurrent measurement period satisfy a congestion threshold. For example,the threshold may be a certain percentage of the number of tiles of thenetwork. If the congestion threshold is satisfied, then the componentindicates that the tiles are congested for the current measurementperiod in block 802. In blocks 803-805, the component loops determiningwhether the pattern of tile congestion over multiple measurement periodssatisfies a tile congestion pattern. For example, a tile congestionpattern may indicate that the tiles are congested when congestion isdetected in any three consecutive measurement periods out of the lastfive measurement periods. Thus, the network may be considered to becongested when tile congestion was detected for measurement periods t-2,t-3, and t-4 (where t represents the current measurement period), eventhough the tile congestion was not detected in measurement periods t andt-1 (that is, a pattern of “11100”). Such a tile congestion patternallows network congestion to be indicated even though the tilecongestion was not detected in the last two measurement periods.However, if tile congestion was not detected in the last threemeasurements periods, then the component would not indicate networkcongestion. Such a pattern prevents the component from alternatingbetween network congestion and no network congestion when the tile stallcount fluctuates slightly above and slightly below the threshold forcongestion. Although network congestion is by patterns, the componentmay be implemented using any logic developed to determine whether acongestion criterion is satisfied to meet the needs of the particularnetwork interconnect, nodes, jobs, etc. In block 803, the componentselects the next tile congestion pattern. In decision block 804, if allthe tile congestion patterns have already been selected, then thecomponent returns an indication that the criterion is not satisfied,else the component continues at block 805. In decision block 805, if theperiod of tile congestion matches the selected pattern, then thecomponent returns an indication that the criterion is satisfied, elsethe component loops to block 803 to select the next pattern. Althoughnot illustrated in a figure, the check NIC congestion criterioncomponent may operate in a manner similar to the check tile congestioncriterion component.

FIG. 9 is a flow diagram that illustrates the processing of a collectperformance information component of a supervisory controller in someembodiments. The component collects performance information, such asstall counts and ejection rate, from the local controllers and storesthe information in a data store. The component may be invoked wheneverinformation is received from a local controller. In block 901, thecomponent retrieves performance information. In block 902, the componentextracts the measurement information. In blocks 903-904, the componentextracts information indicating the number of the network interfacecontrollers and tiles that were stalled during the measurement period.In block 905, the component increments the global count of networkinterface controllers and tiles that were stalled during the measurementperiod. In some embodiments, the local controllers may not sendperformance information (e.g., stall counts) for a measurement period inwhich the performance information is the same as (or very similar) tothat of the previous measurement period. As a result, local controllersonly send performance measurements when performance is changing and neednot send performance information when the overall system is in a steadystate. The component then completes.

FIG. 10 is a flow diagram that illustrates the processing of a stalldetector component of a local controller in some embodiments. Thecomponent may be invoked every measurement period to determine thenumber of network interface controllers and the number of tiles that arestalled. In block 1001, the component invokes the generate NIC stallcount component. In block 1002, the component invokes the generate tilestall count component. In block 1003, the component reports any changein the counts to the supervisory controller and completes.

FIGS. 11 and 12 illustrate processing for NIC stall counts. Although notillustrated in the figures, the processing for tile stall counts issimilar to the processing for the NIC stall counts. In some embodiments,a local controller may collect stall counts on a more granular basisthan a per-tile basis. For example, each tile may support virtualchannels, and the stall counts may be on a per-virtual channel basis.FIG. 11 is a flow diagram that illustrates the processing of a generateNIC stall count component of a local controller in some embodiments. Thecomponent loops through each network interface controller that iscontrolled by this local controller and determines whether the networkinterface controller is stalled. In block 1101, the component selectsthe next network interface controller. In decision block 1102, if allthe network interface controllers have already been selected, then thecomponent returns, else the component continues at block 1103. In block1103, the component retrieves and stores the count of the number offlits processed (e.g., injected into the network) by the selectednetwork interface controller during the measurement period. In block1104, the component retrieves and stores the count of the cycles duringwhich a flit was delayed (i.e., stalled) during the measurement period.In block 1105, the component invokes the check NIC stall criterioncomponent to determine whether the selected network interface controlleris stalled. In decision block 1106, if the selected network interfacecontroller satisfies the stall criterion, then the component marks theselected network interface controller as stalled in block 1107. Thecomponent then loops to block 1101 to select the next network interfacecontroller.

FIG. 12 is a flow diagram that illustrates the processing of a check NICstall criterion component of a local controller in some embodiments. Thecomponent is passed an indication of a network interface controller anda measurement period. In decision block 1201, if a stall threshold issatisfied for the network interface controller, then the component marksthe network interface controller as being stalled for that measurementperiod in block 1202. In blocks 1203-1205, the component loopsdetermining whether the pattern of stalls for the network interfacecontroller matches a stall pattern. In block 1203, the component selectsthe next stall pattern. In decision block 1204, if all the stallpatterns have already been selected, then the component returns anindication that the criterion is not satisfied, else the componentcontinues at block 1205. In decision block 1205, if the next stallpattern matches the selected pattern, then the component returns anindication that the criterion is satisfied, else the component loops toblock 1203 to select the next stall pattern.

FIG. 13 is a flow diagram that illustrates the processing of acongestion abatement component of a supervisory controller in someembodiments. The component may be invoked every measurement period todetermine whether to start or continue abatement of network congestion.In block 1301, the component invokes the check congestion abatementcriterion component. In decision block 1302, if the criterion issatisfied, then the component continues at block 1303, else thecomponent completes. In block 1303, the component identifies theabatement amount such as the fraction for the pulse width modulation orthe maximum rate of the injection of packets into the network. Thecomponent may determine the abatement amount based on the amount ofnetwork congestion, may use a fixed fraction, a fixed maximum injectionrate, or so on. In block 1304, the component broadcasts an abatementnotification to the local controllers and then completes. Alternatively,the component may broadcast only one start abatement notification when astart criterion is initially satisfied and a stop abatement notificationwhen a start criterion is satisfied, rather than sending periodicabatement notifications.

FIG. 14 is a flow diagram that illustrates the processing of a checkcongestion abatement criterion of a supervisory controller in someembodiments. The component is passed an indication of the measurementperiod and determines whether to start or continue abatement of networkcongestion. Once abatement is started, the component may ensure thatabatement continues for a minimum number of periods. The component mayalso use a different criterion for stopping abatement than used forstarting abatement. In particular, the criterion used for stoppingabatement may be based on significantly less network congestion than isrequired for starting the abatement. The use of different criteriaproduces a hysteresis effect to ensure that the abatement system doesnot toggle rapidly between starting abatement and stopping abatement. Indecision block 1401, if the network is congested for the currentmeasurement period, then the component continues at block 1406, else thecomponent continues at block 1402. In block 1402, the component checksthe abatement stop criterion. In decision block 1403, if the abatementstop criterion is satisfied, then the component continues at block 1404,else the component continues at block 1406. In decision block 1404, ifthe number of periods since abatement was started exceeds the minimumnumber of periods for abatement, then the component continues at block1405, else the component continues at block 1406. In block 1405, thecomponent resets the abatement period count and returns an indicationthat the criterion is not satisfied. In block 1406, the component startsor continues with the abatement, increments the abatement period count,and returns an indication that the criterion is satisfied.

FIG. 15 is a flow diagram that illustrates the processing of acongestion abatement component of a local controller in someembodiments. The component may be invoked every period. The abatementperiods may be the same as or different from the measurement periods. Inblock 1501, the component sets a flag to indicate that the networkinterface controller is allowed to inject packets into the network. Indecision block 1502, if abatement of network congestion is currently on,then the component continues at block 1503, else the componentcompletes. In block 1503, the component sets a timeout corresponding tothe width for the pulse width modulation. In block 1504, the componentwaits for the timeout. In block 1505, the component sets a networkinterface controller to stop injecting packets into the network and thencompletes. The component is then invoked during the next period. Theresult of invoking the component during each period is that whenabatement is on, the flag indicates that injection is allowed throughouteach period and when abatement is off, the flag indicates that injectionis allowed only during the fraction of the period starting withinvocation of the component and the flag indicates that injection is notallowed during the remainder of the period.

FIG. 16 is a flow diagram that illustrates the processing of an identifyjob causing congestion component of a supervisory controller in someembodiments. The component may be invoked every period, less frequently,or on an ad hoc basis. The component identifies a job that is causingthe network congestion based on the ejection rate of packets or flitsfrom a network interface controller to a node executing a portion ofthat job. In block 1601, the component invokes an identify suspect jobscomponent to identify jobs that are suspected of causing the congestion.In block 1602, the component invokes a check for guilty job component toidentify which of the suspect jobs is guilty of causing the networkcongestion. In decision block 1603, if the guilty job is found, then thecomponent reports an indication of the guilty job or jobs in block 1604.The component then completes.

FIG. 17 is a flow diagram that illustrates the processing of an identifysuspect jobs component of a supervisory controller in some embodiments.The component identifies those jobs whose ejection rate for the currentperiod exceeds a threshold. In block 1701, the component selects thenode with the next highest ejection rate starting with the highestejection rate. In decision block 1702, if the ejection rate is greaterthan a threshold ejection rate, then the component continues at block1703, else the component returns. In block 1703, the componentidentifies the jobs executing at the selected node. In block 1704, thecomponent marks the identified jobs as being suspected of causing thecongestion for the current period. The component then loops to block1701 to select the next node.

FIG. 18 is a flow diagram that illustrates the processing of a check forguilty job component of a supervisory controller in some embodiments.The component determines whether suspect jobs for the current periodmatch a guilty criterion. For example, the guilty criterion may be thatthe suspect job has the highest ejection rate over the last fiveconsecutive periods or the last few times that abatement was started.The guilty criterion may also be based on other factors such as numberof nodes on which a job is executing, not being on a list of jobs thatare unlikely to cause congestion, and so on. In block 1801, thecomponent selects the next suspect job for the current period. Indecision block 1802, if all the suspect jobs have already been selected,then the component returns an indication of whether a guilty job hasbeen identified. In block 1803, the component checks the guiltycriterion for the selected suspect job. In decision block 1804, if theguilty criterion is satisfied, then the component continues at block1805, else the component loops to block 1801 to select the next suspectjob. In block 1805, the component marks the selected job as beingguilty. In block 1806, the component sets the guilty status to found andloops to block 1801 to select the next suspect job.

FIG. 19 is a flow diagram that illustrates the processing of a sendejection count component of a local controller in some embodiments. Thecomponent may be invoked every measurement period or other period (e.g.,during congestion). In block 1901, the component retrieves the count offlits received by each node connected to the local controller for thecurrent period. In block 1902, the component sends the counts to thesupervisory controller and then completes.

Although the subject matter has been described in language specific tostructural features and/or acts, it is to be understood that the subjectmatter defined in the appended claims is not necessarily limited to thespecific features or acts described above. Rather, the specific featuresand acts described above are disclosed as example forms of implementingthe claims. In some embodiments, various parameters of the detectionsystem such as thresholds, criteria, patterns, lengths of periods, andso on can be determined empirically based on analysis of thecharacteristics of the network interconnect including architecture,speed of components, clock rates, and so on. The parameters may bedifferent for the different types of network components (e.g., tiles ornetwork interface controllers). The supervisory controller may provide auser interface through which an administrator may tailor the detectionsystem by defining functions for use in evaluating such parameters. Suchfunctions may be distributed to the various controllers that implementthe detection system. In some embodiments, the various controllers mayperform simple data collection and forwarding without analysis of thecollected data to the supervisory controller for analysis. In someembodiments, the congestion detection system may collect performancemeasurements at the router level, rather than for components of arouter. The congestion detection system may detect congestion at variousconnection devices for sending data through a network and betweennetworks. The connection devices may include routers, switches, networkswitches, switching hubs, switching devices, routing devices, networkrouters, packet switches, connectors, sub-components of such connectiondevices, and so on. The components of a network may be connected viawired or wireless connections. Although the data routed through thenetwork is described as being organized as packets with flits, the datamay be organized in other ways (e.g., packets without any subdivisioninto flits, packets with a fixed number of sub-packets or flits, orfixed-sized packets). Accordingly, the invention is not limited exceptas by the appended claims.

We claim:
 1. A method for detecting congestion in a network of nodesconnected via routers for routing packets between nodes, the networkhaving network components, the method comprising: for each networkcomponent, generating performance measurements based on whether a packetwas delayed at the network component during a measurement period; anddetermining based on the generated performance measurements whether thenetwork component satisfies a stall criterion; for each of a pluralityof the network components, obtaining an indication that the networkcomponent satisfies the stall criterion; determining based on theobtained indications of the network components that satisfy the stallcriterion whether the network satisfies a network congestion criterion;and upon determining that the network satisfies the network congestioncriterion, indicating that the network is congested, wherein each packetincludes one or more flits, each network component has a componentperiod during which the network component can forward a flit, and theperformance measurements indicate a number of flits forwarded by thenetwork component during the measurement period and a number ofcomponent periods during the measurement period in which a flit wasdelayed at the network component; and wherein the stall criterion issatisfied for a network component based on whether a ratio derived fromthe number of flits forwarded during a measurement period and the numberof component periods during the measurement period in which a flit wasdelayed satisfies a stall threshold.
 2. The method of claim 1 whereinthe network component is a network component of a router.
 3. The methodof claim 1 wherein the network component is a network interfacecontroller connecting a node to a router.
 4. The method of claim 1wherein the stall criterion is satisfied for a network component basedon performance measurements for multiple measurement periods.
 5. Themethod of claim 1 wherein the network congestion criterion is satisfiedwhen the number of network components that are stalled satisfies anetwork congestion threshold.
 6. The method of claim 5 wherein thenetwork congestion threshold is based on a combination of a number oftiles that are stalled and a number of network interface controllersthat are stalled.
 7. The method of claim 1 wherein the networkcongestion criterion is satisfied based on performance measurements formultiple measurement periods.
 8. The method of claim 1 wherein thedetermining of whether the network satisfies the network congestioncriterion is performed by a supervisory controller that collects via anout-of-band network connection information from local controllers thatcollect performance measurements from the network components.
 9. Themethod of claim 8 wherein the performance measurements for a networkcomponent are collected by a local controller accessing one or moreregisters of the network component.
 10. The method of claim 8 whereineach local controller determines whether the stall criterion issatisfied by the network components from which the local controllercollects performance measurements.
 11. The method of claim 10 whereinthe local controllers suppress the sending of the count when the countis unchanged from the previous measurement period.
 12. The method ofclaim 10 wherein the local controllers send to the supervisorycontroller a count of network components that satisfy the stallcriterion during each measurement period.
 13. A computer-readable mediumthat is not a transitory, propagating signal storing computer-executableinstructions for controlling a computing device to detect congestion ina network of nodes connected via connection devices, each connectiondevice having multiple network components, the computer-executableinstructions implementing a method comprising: for each connectiondevice, receiving performance measurements for the network components ofthe connection device indicating whether data is delayed at the networkcomponents, wherein the data is organized into packets and each packetincludes one or more flits, a connection device has a connection deviceperiod during which the network components of the connection device canforward a flit, and the performance measurements indicate a number offlits forwarded by the network components of the connection deviceduring the measurement period and a number of connection device periodsduring the measurement period in which a flit was delayed at a networkcomponent of the connection device; determining based on the receivedperformance measurements of the network components of the connectiondevices whether the network components of the connection devices satisfya network congestion criterion, the determining including determiningwhether a network component of a connection device satisfies a stallcriterion based on whether a ratio derived from the number of flitsforwarded during a measurement period and the number of connectiondevice periods during the measurement period in which a flit was delayedsatisfies a stall threshold; and upon determining that the networkcongestion criterion is satisfied, indicating that the network iscongested.
 14. The computer-readable medium of claim 13 wherein thenetwork is congested when the number of network components of theconnection devices that are stalled satisfies a network congestionthreshold.
 15. The computer-readable medium of claim 13 wherein theconnection devices are routers and the network components are tiles ofthe routers.
 16. A computing system for detecting whether a connectiondevice of a network is congested, the network having nodes connected viathe connection devices for sending packets between nodes, the computingsystem comprising: local controllers that generate performancemeasurements for network components of the connection devices of thenetwork indicating that data was delayed at a network component of aconnection device during a measurement period and forwards the generatedperformance measurements to the supervisory controller; and asupervisory controller that receives the performance measurementsforwarded from the local controllers and determines whether the networkis congested based on the received performance measurements of aplurality of network components of a plurality of connection deviceswherein a packet of data includes one or more flits, each networkcomponent has a component period during which the network component canforward a flit, and the performance measurements indicate a number offlits forwarded by the network component during the measurement periodand a number of component periods during the measurement period in whicha flit was delayed at the network component, and wherein a stallcriterion is satisfied for a network component based on whether a ratioderived from the number of flits forwarded during a measurement periodand the number of component periods during the measurement period inwhich a flit was delayed satisfies a stall threshold, and wherein thelocal controllers and the supervisor controller communicate via aconnection that is not part of the network.
 17. The computing system ofclaim 16 wherein a local controller determines whether the generatedperformance measurements indicate that a connection device satisfies astall criterion and reports to the supervisory controller whetherconnection device is stalled.